Area weighted regridding

The PR adds area weighted regridding to ```iris.experimental.regrid```.

It is not the finished product:

1. More testing is needed, 
2. I need to handle aux factories so the surface altitude is regridded using area weighting.
3. Removal of debug print statements.

I have put it up so @bblay can begin the review process.

Note the travis failures are due to an expectation that ```is_contiguous``` will be modified (#459), partial aux factory support and a dependency on numpy 1.7 to support a tuple as the axis argument.